1. Field of the Invention
This invention relates generally to degradation of xanthan molecules and, more specifically to degradation of xanthan molecules at moderate to high temperatures. In particular, this invention relates to a method for treating wells, subterranean formations, and other applications using a microbial xanthanase which is active at high temperatures and/or downhole conditions. This invention also relates to a soil bacterium capable of producing the microbial xanthanase.
2. Description of the Related Art
Polysaccharides represent one typical type of polymer used in drilling, completion, and remedial operations. Among other things, polysaccharides may be used as a part of fracturing gels for hydraulic fracturing, to thicken drilling fluids, to control fluid loss, and as a part of gravel packing and frac pack fluids. Polysaccharides may also be used in sand control fluids, blocking gels, and completion fluids. Filter-cakes, filtrate invasion, and other similar types of formation damage are phenomena which often occur during various procedures performed within a wellbore using polysaccharide polymers including, but not limited to, drilling, completion, workover, and stimulation procedures.
For example, during drilling operations polysaccharide-based fluids containing high concentrations of clays, such as bentonite, which are typically used for lubrication and cuttings transport. These fluids are known to cause damage to the permeability of the near wellbore area due to leakoff and mud cake or filter-cake deposition on the face of production zones. A filter-cake is typically a dense and nearly water insoluble residue that may, among other things, serve to reduce the permeability of a subterranean formation. Filter-cakes may be formed when gel fluids leak into a formation matrix through rock pore spaces. In this case, the pores of a formation act as filters which permit fluid to leak into the matrix while filtering out the gel. This causes a layer of filtered gel to deposit on the face of the matrix, plugging the formation. In other cases, incomplete gel degradation may also result in the formation of a polysaccharide filter-cake. A filter-cake may contain precipitates, such as silicates from drilling muds or residues derived from polymer containing gelable fluids. A filter-cake typically interferes with the production of a subterranean formation by filling the rock matrix pores, thereby inhibiting the flow of fluids from the matrix. Filter-cakes may also serve to restrict flow in hydraulic fracture proppant beds and other flow channels.
In addition to formation damage caused by filter-cake formation, well treatments and drilling operations utilizing polysaccharides may also result in the deposition of relatively viscous fluids and residues within a productive zone which create damaging conditions similar to those created by filter-cakes. Thus, formation damage may be related to filter-cake, filtrate, residues and other related materials that invade a productive zone. As a result, it is often necessary to apply stimulation treatments to bypass this drilling fluid damage in such intervals.
In the practice of drilling wells in horizontal or highly deviated configurations, as well as multi-lateral completions, wells are drilled in order to contact more hydrocarbon-bearing pay zone area within a single well in order to maximize productivity. By "deviated" it is meant that at least a portion of a wellbore has an angle of between about 0.degree. and about 90.degree. from the vertical, and by "highly deviated" it is meant that at least a portion of a wellbore has an angle of between about 45.degree. and about 90.degree. with respect to the vertical. Such wellbores often penetrate thousands of feet of productive zone as opposed to the tens to hundreds of feet contacted in vertical well configurations. Consequently, productivity damage created by polysaccharide filter-cakes, residues, and/or filtrates is exacerbated over long productive intervals within these types of well configurations.
Insufficient degradation of polysaccharide-induced damage may significantly impede flow capacity at the wellbore wall. Such reduced flow capacity may result in significant reduction of the productivity or infectivity of vertical and horizontal wells. In horizontal or highly deviated wells in particular, it is important that formation damage from drilling fluid leakoff and filter-cake deposition be mitigated or eliminated to realize the full potential of these types of completions. Furthermore, obtaining zonal isolation with cement in the presence of a filter-cake and/or residue is often difficult because these layers interfere with formation of a pressure seal between a wellbore and a production pipe string. This may occur when the presence of filter-cakes or residues between a borehole wall and pipe string blocks circulation or placement of cement in the annular area between the borehole and the casing or between two strings of pipe, thereby creating pockets of filter-cake, residue, or other non-cement materials which may result in fluid communication in the annular area between the pipe string and borehole wall or between the two pipe strings that the cement is supposed to isolate. In a completed well this may result in a loss of hydraulic integrity due to fluid movement through a filter-cake, residue layer, or other pocket underneath the cement sheath of a completed well.
A common approach to minimizing formation damage from filter-cake, filtrates, and residues has been to apply acid or strong oxidative breaker systems to dissolve filter-cake solids and polymers. A typical wellbore treatment to remove such damage consists of hydrochloric acid solutions, solutions of lithium or sodium hypochlorite, or highly concentrated solutions of conventional oxidizers like ammonium persilofate or perborate. Although acids and oxidative solution washes appear to perform reasonably well in a laboratory environment where contact of filter-cake damage with a reactive solution is easily achieved, application of these solutions may not be effective for removing the damage in horizontal intervals. For example, field experience has demonstrated that acids and oxidative solutions used to remove mud filter-cake damage have proven relatively ineffective based upon well performance. The problem is particularly evident when such treatments are applied in extended length openhole intervals. One rationale that has been proposed to explain this problem is the difficulty of contacting filter-cake materials with these reactive solutions. For example, studies have indicated that polymer coated carbonate particles used for weighting and fluid loss control may be resistant to acid attack and prevent complete removal of a filter-cake. See Burnett, D. B. "Using a Physical Wellbore Model to Study Formation Damage Problems in Well Completions," paper SPE 27393 presented at the 1994 International Symposium on Formation Damage Control, Lafayette, February 9-10.
Additional concerns regarding the use of acidic or oxidative cleanup treatments include the reactivity with tubulars which may result and elevated iron concentrations being injected into the reservoir in a manner which may promote sludging problems.
A typical polysaccharide employed in well fluids is xanthan. Xanthan containing fluids are known to cause damage to the permeability of the near wellbore area due to leakoff and mud or polymer filter-cake buildup on the formation faces in the same manner as other polysaccharides, such as celluloses and starches. Xanthan is a biopolymer that may be produced by a bacterial fermentation. It is a heteropolysaccharide of which the structure consists of a linear chain of D-glucose units that are bonded together by 1, 4-.beta.-glucosidic linkages with trisaccharide substituents attached to the glucose backbone by .beta. 1-3 glycosidic or mannosidic linkages. Xanthan may be used in a variety of industrial applications, for example, as described by Jeanes, "Applications of Extracellular Microbial Polysaccharide-Polyelectrolytes: Review of Literature, Including Patents," J Polym. Sci., Polym. Symp. No. 45, pp. 216-221, 1974; and in, for example, U.S. Pat. No. 4,119,546. Typical well applications include, but are not limited to, those mentioned above, most typically as a brine thickener in drilling muds and workover fluids, as a viscosifier in hydraulic fracturing and cementing, as a gel blocking agent in gravel packing and frac packing operations, in secondary and tertiary recovery operations, and in non-petroleum applications such as a clarifier for use in refining processes. As previously described, conventional acid and oxidizer treatments to reduce polymeric damage are typically ineffective to remove or mitigate xanthan damage due to the resistance of xanthan towards oxidizers and acids. Although well treatments using xanthan-specific enzymes have been proposed to treat xanthan polymer damage, these treatments employ enzymes that are typically not effective at temperatures greater than about 150.degree. F. Because many wells have downhole temperatures exceeding 150.degree. F., proposed enzyme treatments for removing xanthan damage would be ineffective in many wells having temperatures exceeding this level.
In some wellbore related applications, it is desirable to reduce the viscosity of xanthan-containing fluids. For example, during hydraulic fracturing, a sand laden fluid is injected into a wellbore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gelation of suitable polymers, such as xanthan, which in this capacity are known as fracturing gels. The gelled fluid can be accompanied by a propping agent which results in the placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the wellbore into the formation being treated once the fracturing fluid is recovered. Propping agents include a wide variety of material and may be coated with resins. The gel fluids may also contain other conventional additives common to the well service industry such as surfactants, and the like.
In another example, production from wellbore operations must cease temporarily to perform auxiliary procedures called workover operations. The use of temporary blocking gels, also formed by gelation of appropriate polysaccharides such as xanthans, produces a relatively impermeable barrier across the production formation. These gels may also be used as diverting agents during stimulation treatments. In this capacity, the gels are typically pumped into a formation ahead of a stimulation fluid, such as acid. The gels selectively enter the more permeable zones of the formation where they create a relatively impermeable barrier across the more permeable zones of the formation, thus serving to divert the stimulation fluid into the less permeable portions of the formation. After such a treatment the gel barrier may be broken internally or externally to allow production from, or injection into, both zones of the formation. In other cases, such blocking gels may be used in a similar manner to block the production or injection of water in secondary recovery operations by gel treatments of injection and/or production wells.
In still another example, uncrosslinked xanthan-containing polysaccharides are used thicken fluids and control fluid loss. In this capacity they may be used with proppants, such as sand control fluids and completion fluids, such as those for gravel packing. Gravel packing controls sand migration from unconsolidated or poorly consolidated formations through the placement of a gravel pack around a slotted or perforated liner or screen liner inserted at a specific location within a perforated wellbore. The "gravel" is usually sand or a very fine gravel that excludes the formation sand from entering the wellbore. Xanthans are typically used to thicken the fluids in order to properly pack gravel into the perforations of the wellbore. Although unthickened slurries pack an annulus well, the sand compacts quickly and may not have sufficient time to flow into and completely pack the perforations.
In the above examples the viscosity of xanthan-containing fluids, whether crosslinked or not, is most often desirably reduced at the end of an operation. At the end of fracturing or workover operations for example, the gels are degraded and the fluids are recovered. Gel fluids are recovered by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion of gels is referred to as "breaking" and is often accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
A similar reduction of the fluid viscosity of uncrosslinked, xanthan-containing fluids occurs at the end of completion operations. For example, at the end of gravel packing, the viscosity is reduced to allow the settlement of sand to properly pack the annulus. Therefore in this disclosure, "breaking" refers to the reduction of viscosity of a xanthan-containing fluid, whether crosslinked or uncrosslinked, to a low value such that it flows from the formation under the influence of formation fluids and pressure.
In addition to the importance of providing a breaking mechanism for the fluid which facilitates recovery of the fluid and resumes production, the timing of the break is of great importance. Gels that break prematurely can damage the production zone through the leak-off of contaminating materials into the production formation. If the viscosity is reduced prematurely during gravel packing, the sand settles before being properly placed within the wellbore and perforations, thus contributing to the problem of sand within the wellbore.
On the other hand, fluids that break too slowly can cause slow recovery of the fluid from the production formation. Slow recovery delays the resumption of the production of formation fluids and can cause improper packing the annulus during gravel packing. Incomplete gel degradation causes a build up of residue which interferes with production from the formation.
For purposes of the present application, premature breaking means that the viscosity diminishes to an undesirable extent prior to the end of the operation. In the typical case, it is desirable for a viscosity to remain in the range from about 60% to about 100% for the length of time required to complete the operation. However, in other cases, lower viscosities during this time are acceptable. Since some operations require extended periods of time before completion, the fluids should be capable of remaining appropriately viscous during that time period. In the laboratory setting, viscosity is measured using a rotational viscometer such as a Fann "35VG" meter or a Brookfield "DVII" digital viscometer.
For practical purposes, the viscosity of the xanthan-containing fluid should be completely reduced within a specific period of time after completion of the operation. This period of time depends on the temperature of the formation. Optimally, a gelled fluid breaks when the operation concludes. A completely reduced fluid means one that may be flushed from the formation by the flowing formation fluids and/or formation pressures. Desired characteristics of a substantially broken, uncrosslinked gel varies according to the permeability of a particular formation. However, for most formations such a broken gel regains greater than about 65% of the initial permeability of a formation sample using a gel damage permeability test.
Enzyme systems are known to degrade the types of polysaccharides used in fracturing and blocking gels as well as in other applications. Enzyme breaker systems have been designed to break gelled fracturing and blocking fluids used in the industry as well as filter-cakes. See for example U.S. Pat. Nos. 5,224,544; 5,247,995; 5,201,370; 5,562,160; and 5,566,759. Xanthan enzyme systems described in these references degrade xanthan-containing fluids at low to moderate temperatures of up to about 150 .degree. F. However, these enzyme systems are less effective at temperatures above about 150.degree. F.
Xanthan-based well fluids are also stored and maintained on the surface. For example, xanthan containing drilling mud may be stored and maintained within a reserve pit, mud pit, or frac tank. In such cases the drilling mud typically contains a relatively large solids content, including drilled solids and solid weighting materials. After a well is drilled or a remedial well operations is completed, large volumes of xanthan containing drilling materials may remain on the surface within reserve pits or other similar storage areas. In order to remove these fluids after a well operation, the solid materials must be separated from the liquid phase. This is often difficult due to the presence of polymeric viscosifiers such as xanthan. Separation typically requires processing through separation equipment including cyclone separates, decanter centrifuges, shakers and the like, as well as the use of a large volume of water.
Xanthan-based fluids are also used in high temperature non-well applications. For example, xanthan may be used in industrial processes such as in clarification steps of a refining process. In this and other similar applications, xanthan-based filter-cakes and residues may accumulate on porous permeable media or other areas of process equipment. These filter-cakes and residues need to be degraded and removed periodically or on a continuous basis. As in well applications, xanthan-based filter-cakes and residues are difficult to remove under high temperature process conditions, and enzyme systems are typically limited to temperatures of about 150 .degree. F. In the past, steam is one method that has been employed to remove xanthan filter-cakes and residue from process flow equipment, such as in refining processes.
Consequently, a need exists for effective methods and compositions for removing xanthan-based damage from a well. In particular, a need exists for a method for stimulating productivity of a subterranean formation damaged by drilling and other fluids containing xanthan molecules at temperatures exceeding about 150.degree. F. A need also exists for a method of improving cementing and other well treatment performance by removing areas of xanthan-based filter-cake and/or residues at temperatures exceeding about 1 50.degree. F. A need also exists for an enzyme breaker or system which is effective to degrade xanthan-containing fluids at temperatures exceeding about 150.degree. F. Further, a need exists for a method of degrading xanthan-based fluids, filter-cakes and residues in process flow systems having similar high temperatures.